A footing is the part of a structure that transfers loads to the ground upon which the structure stands. Like your feet, it provides stability and distributes weight, so it is an extremely important component of a building. If you’re wondering about the types of footings in construction and which may be best for your project, we’re here to help!
A footing may be shallow or deep, concentric or eccentric, isolated, raft, strip, pile, or another type. The type of footing depends on soil conditions, frost levels, the magnitude of the structure, and all loads required, plus other factors. Regardless of type, the purpose is the same.
In this article, we’ll explain what footings are and discuss the different types used in construction. We’ll explore the differences between footings and foundations and identify which type of footing is best for different structures. Our aim is to provide you with the information you need for your building project.
- What Are Footings in Construction?
- Different Types of Footings in Construction
- What’s the Difference Between Footing and Foundation?
- Which Type of Footing Is Best For
What Are Footings in Construction?
A footing is usually the first part of a structure to be built. It may sit on top of megalithic stone or rest inches or feet deep on load-bearing soil or ground. It is the bottom part of a foundation. A footing transfers structural loads to the ground underneath, provides stability to the foundation, helps prevent it from settling, sinking, or cracking, provides a level surface upon which to build the structure, and minimizes movement.
The type of foundation depends upon soil type and condition, how deep the safe load-bearing strata are, and the type and magnitude of the structure being erected. The depth of the footing also depends on frost levels too.
There are different types of footings used in construction, most today are made of concrete with steel reinforcement. However, stone, brick, and wood are also used depending on the type of structure and expected loads.
Some footings provide individual support to a structural component. Others support large sections, and still others carry whole structures. The type of structure, climate, depth of the load-bearing strata, and lay of the land all influence the type of footing.
There are individual, combined, strip, and raft or mat types of footings, as well as others. However, regardless of type, they all serve the same function; to provide stability and support while distributing structural loads to the surrounding ground.
Different Types of Footings in Construction
Footings are used for single-family residences and multi-family dwells, commercial, high-rises, industrial complexes, and bridges. They are often used for barns, sheds, garages, lamp posts, and towers, as well as other structures.
There are many types of footings. Some have specific functions and others are more generic, and the terms used to identify them vary across the industry and from location to location too.
A footing may be shallow or deep depending on the expected structural loads and the load-bearing capacity of the soil or ground strata. Shallow footings are commonly used when the horizontal area covered by a structure is greater than its vertical height. It distributes loads laterally while a deep or pile footing distributes it vertically.
In this section, we’ll identify different footings and explain what they are and how they work so that if you know them by a different name, you’ll know what we’re talking about. Plus, we’ll identify some common uses.
1. Spread / Isolated / Individual Footing
A spread footing is one that spreads a structural load over a broad soil base so it doesn’t exceed its safe bearing capacity. It often isolates or separates a load from other load-bearing components, and is commonly used to support the loads from individual posts or columns. Hence, its three-fold name.
The pad usually is concrete with rebar and may include an interconnected reinforced column base too. It is commonly square but may be circular or rectangular. It is also known as a simple, flat, plain, or pad footing.
It is often used to support beams in residential construction that require one or more posts or columns with a concrete pad underneath to distribute the loads. Sloped and stepped footings are also isolated or individual footings.
2. Combined Footing
A combined footing refers to a footing that combines the load from two or more posts or columns. The columns may be close together and their footings would overlap, so a larger rectangular or trapezoidal footing or pad is used. A combined footing is also used if the bearing capacity of the soil is too low and a larger support area is needed.
Another situation for using a combined footing is if the base of the column will be too close to the property line to permit a regular footing placement. In that situation, an eccentrically loaded combined footing may be used to spread the load within the property boundary.
3. Wall / Strip Footing
A strip footing is a continuous footing commonly used under a wall, so it is often referred to as a wall footing. Usually made of reinforced concrete, it distributes the structural loads across the bearing strata. The width and thickness of the footing and the number and placement of rebar depend on the structural loads and the load-bearing capacity of the soil.
The footing’s depth in the ground also depends on soil strength, topographical considerations, design, structural requirements, and frost levels. Strip or wall footings are often found in residential, commercial, and industrial construction.
4. Pedestal Footing
A pedestal footing is a concrete or stone compressive structural component used to uniformly transfer live and dead loads from columns, posts, or statues to a footing underneath it. Without the pedestal, the loads on a column may not concentrically transfer to the footing.
The pedestal ensures the load and footing axis are the same so the center of gravity is maintained, and the loads are uniformly transmitted. A pedestal may be a separate member or part of the poured footing. It often protrudes above the floor line, thus preventing wood or steel columns or posts from coming in contact with soil or water.
5. Strap Footing
A strap, shoe, or cantilevered footing is an eccentric footing and refers to two spread or individual footings that are joined with a reinforced concrete strap or narrow footing-like strip. The individual footings or pads each support a column or post to transfer the loads to the ground, however, the connecting strap has no load transfer purpose.
The strap helps to ensure the center of gravity of the two columns’ combined loads continue through the center of gravity of the two footings. So, the load distribution pressure is uniform for each footing, even if one column isn’t centered on its footing pad.
6. Raft or Mat Footing
A raft or mat footing is a shallow foundation that combines a reinforced cement concrete (RCC) slab and beams that cover the whole area of the structure. Dense concrete is often used to construct heavy structures in areas where the ground behavior is unpredictable, such as low-strength soil, filled ground, or marshy areas.
The footing slab typically ranges from 4” to 20” in thickness and may have built-in support beams above or below the slab. It is often used for multi-level structures, warehouses, and large commercial or industrial structures, especially where a pile foundation isn’t feasible.
7. Pile Footing
Pile foundations are used when the supporting ground or soil strata aren’t strong enough to support the expected loads of the structure if using a foundation on a shallow footing. The piles provide support through two different methods – end bearing or friction bearing. They are known as deep foundations and provide support vertically instead of laterally.
Shallow footings are often supported by piles for greater support. Piles may be wood, steel, precast concrete, or poured concrete and are used to support bridges, wharves, water tanks, skyscrapers, and other structures.
End Bearing Piles
End bearing piles are drilled or driven through weak or loose soil or ground strata until they reach solid rock or strong weight-bearing soil strata. Structural loads are transmitted through the pile to the bearing strata at its end.
These are long-lasting or permanent supports and are the most commonly used piles in construction. They are frequently used for high-rise and bridge construction as they minimize settling.
Friction or floating piles are driven through the soil strata and rely on friction to distribute the load weight evenly to the surrounding soil. They are commonly used when load-bearing soil or rock is too deep or it’s not economically viable to reach.
Friction piles can be wood, metal, or concrete but are usually cylindrical to maximize the load transfer surface. They are commonly used for wharves and bridges, and to support large structures on poor supporting soil like sand.
In places like Dubai where sandy soil goes deep, they were used to support skyscrapers like the 2,722-foot-high Burj Khalifa.
8. Stepped Footings
A stepped footing is a stepped or tiered spread, isolated, or individual footing used to raise a column or post above ground level. It may sit upon a plain cement concrete pad and have two or three progressively smaller pads centered on top forming a 4-sided step footing that brings the top of the footing to the required elevation for the column base. They are usually limited to a depth of 36”.
This type also decreases the amount of concrete, thus reducing the weight and costs. They were a common feature in early multistory residential construction but are not often used today.
A stepped footing can also refer to a cost-effective way to support a foundation built on a slope. The footing is a series of reinforced overlapping horizontal concrete steps that follow the slope of the load-bearing strata. It is often used for residential construction where the ground slopes from the horizontal to create a level base for foundation walls.
9. Sloped footings
A sloped or trapezoidal footing has a large square or rectangular base and a smaller similar-shaped top. It is designed and carefully constructed so all sides slope at 45° between the base and top, forming a flat-topped square-based pyramidal-like footing.
The shape helps to center the transferred loads between the column and base, so the load and footing axes are the same. A more modern rendition of the stepped column footing, it uses less concrete and is a common feature in multilevel residential construction.
10. Concentrically Loaded Footing
A concentrically loaded footing is one in which the loads acting on the base are centered on the footing area, so the load axis and footing axes are the same. The loads are distributed uniformly downwards from the structure and upwards from the ground. Most isolated, spread, individual, or other types of footings are concentrically loaded footings.
11. Eccentrically Loaded Footing
An eccentrically loaded footing is one that is not concentrically loaded. It is an unconventional design where the load and footing axis are not the same. However, the maximum pressure under the footing mustn’t exceed the allowable bearing pressure of the soil or ground strata.
A shoe or strap footing is an eccentric footing and is commonly used when no setback at a property boundary is required. They are typically used in downtown cores for high-density residential or commercial structures.
It allows one column to be centrically placed on a pad within the building plot with a connecting strap helping to balance the load axis of a column at or near the edge of the adjoining pad without it being centered on its footing.
What’s the Difference Between Footing and Foundation?
The terms footing and foundation are two of the most basic words in construction. They are parts of the substructure, so are typically below ground level. All footings are foundations; however, it can’t be said that all foundations are footings.
A footing is a thick, wide, level component of a foundation used to support and spread structural loads directly to a wide soil or ground area. Its width and thickness depend upon the size and type of foundation and the bearing strength of the soil.
A footing is usually part of a shallow foundation and is made of steel-reinforced concrete. The footing is the load-transfer part of the foundation and is directly in contact with the ground. It provides stability to the structure and prevents settling.
A foundation is a broad term used to identify the lowest parts of a structure. It supports and transfers above ground or superstructure loads to the ground below. A foundation may be deep or shallow and is often in contact with soil.
However, the foundation may be composed of walls and columns that support the superstructure and extend below grade level to sit upon footings that stabilize the structure and distribute the loads to the ground below. The foundation is like the legs supporting the upper body, and the footings are the feet that keep everything upright and spread the body’s weight to the ground.
Which Type of Footing Is Best For
The type of footing often depends on the size of the structure it will stabilize and support, the load-bearing capacity of the ground or soil underneath, foundation depth, and frost depths. The depth of the water table, distance to adjacent structures, and the magnitude of the loads being supported by foundation walls, columns, or posts also need to be factored in too.
Additionally, local building codes should always be reviewed before building any structure to ensure there are no issues with compliance.
The best type of footing for one structure in one area may differ from that needed in a different area. Those identified below are the most commonly used for the kind of structure identified.
A house with a full basement or perimeter foundation wall uses a strip or wall footing that typically is below local frost levels. If the structure requires interior beams to support structural loads, there will usually be one or more isolated or spread pads to support columns or posts.
The house may be built without a basement using a slab on grade or raft-type footing. Homes built on uneven ground may use a post and beam style foundation with the posts resting on isolated footings. In some areas, due to poor soil strengths, high water tables, or storm surges a pile foundation may be used to support structural loads and raise the building above potential water hazards.
The size and elevation of a deck often determine the type of footing best suited for it. Ground hugging or near-ground decks typically use isolated footings or deck blocks to transfer loads to the ground below.
The deck block may sit upon a larger cast concrete pad to provide more stability on low-bearing soil. Decks at greater elevations may use poured or prefab concrete piers to support posts, or may use isolated footings to provide stability and load transmission.
A porch is typically a roofed platform attached to a house and often provides shelter over an entrance to the dwelling. As such, it must meet building code requirements. The footings need to reach below local frost levels and may be isolated, pier, grade beam or strip, slab or raft, or a combination of different types depending on the size and weight being supported.
A pole barn requires a footing that will support vertical loads transferring downward from the posts. The footings must be able to support the roof, walls, and any wind and snow loads common to the area. It also needs to be below local frost levels.
To prevent the poles from pushing through the footing, it needs to be of thick, high-pressure reinforced concrete. Isolated pads with special anchors are commonly used, as are wall or strip footings. In some areas, slab-on-grade or raft footings are used depending on soil strengths and structural use.
The poles or columns must be secured to the concrete with special brackets. Pedestal footings are another option as they raise wooden components above soil and moisture contact.
The type of brick wall and its purpose often determine the kind of footing required. A reinforced concrete strip or wall footing is commonly used to support a brick wall. However, if the footing needs to be deep due to frost levels or to reach stable soil strata, concrete piling or pier footings can be used.
A strip footing, stem wall, or foundation can then be built on the pier/piling footings to support the brick wall.